1. Field of the Invention
The present invention relates to a light deflector which incorporates a rotating polygonal mirror having a plurality of mirrors around it to deflect a light beam and scan an image formation member or a recording member with the deflected beam.
2. Discussion of the Related Art
Generally, in an image reading apparatus which scans an image formation member with a light beam radiating from a light source such as a laser to read an image on the image formation member, or in an image recording apparatus which scans a recording medium with a light beam modulated by an image signal or character signal to record an image, a rotating polygonal mirror having a plurality of mirrors on its side faces is used as a means of scanning with the light beam.
One example of this type of light deflector is known: a light deflector comprising a driving shaft bearing, in particular preferably a dynamic pressure air bearing which has a shaft inserted into a sleeve, one of the sleeve and shaft being a rotation member and the other a fixed member and having a permanent magnet fixed on the rotation member, an electromagnetic circuit made by winding a coil on an iron ring installed near the fixed member, wherein the permanent magnet and the electromagnetic circuit generate rotation torque and the electromagnetic circuit functions as a magnetic bearing supporting the rotation member in an axial direction.
FIG. 6 illustrates the construction of an image recording apparatus employing this type of light deflector. 61 is a drive motor, 62 is a rotation polygon mirror, 63 is one of the mirrors constituting the rotating polygonal mirror 62, 64 is a mirror cover, 65 is an aperture for letting the light beam in and out, 66 is a laser, 67 is a collimator lens, 68 is a converging optical system and 69 is a light-sensitive member.
In the figure, the rotating polygonal mirror 62 is rotated by the drive motor 61 in the direction indicated by an arrow (A). A light beam radiates from the laser 66, which may be a semiconductor laser, a gas laser or the like, is modulated by a modulation means (not shown in the figure) with an image signal etc., and is incident on the mirror 63 of the rotating polygonal mirror 62.
The light beam reflected by the mirror 63 of the rotating polygonal mirror 62 is incident on the light-sensitive member 69 through the converging optical system 68.
As the rotating polygonal mirror 62 rotates in the direction indicated by the arrow (A), the reflected light beam is deflected in the direction indicated by an arrow (B) and the light-sensitive member 69 is scanned with it. At the same time, the light-sensitive member 69 is moved to perform a slow scan by rotating in the direction indicated by an arrow (C), whereby a two-dimensional image is formed on the light-sensitive member 69. The light deflector may have also a mirror cover 64 with aperture 65 for letting the light beam in and out to prevent dust from the air attaching to the mirrors 63 of the rotating polygonal mirror 62.
FIG. 7 is a sectional view illustrating the construction of the light deflector used in the above-described image recording apparatus or an image reading apparatus.
In the figure, 1 is a fixed shaft, 1-1 are grooves for forming a dynamic pressure air bearing, 2 is a housing, 3 is a rotation sleeve, 4 is a mirror flange, 5 is a magnet yoke, 6 is an inner magnet, 7 is an outer magnet, 8 is a stator core, 9 is a substrate fixing stud, 10 is a circuit substrate on which a drive control circuit or the like is mounted, 11 is a magnetic field detecting element, 12, 14, 17, 23 and 25 are screws, 13 is a stud for fixing the stator core 8, 62 is a rotating polygonal mirror, 16 is a cap flange, 18 is a very small aperture formed in the cap flange 16, 19 is an air reservoir, 20 is a spacing between the rotation sleeve 3 and fixed shaft 1, 21 is a housing adaptor for mounting the light deflector on the frame of the apparatus, 22 is a damper, 24 is a collar and 63 is a mirror facet.
In the figure, one end of the shaft 1 (in this figure, the lower end of the shaft) is fixed to the housing 2. On the surface of the fixed shaft 1 are formed grooves 1-1 for forming the dynamic pressure air bearing which functions as a radial bearing, which prevents shifting of the center of rotation from its predetermined position when the fixed shaft 1 is affected by a force at a right angle to it.
A rotor portion constituting a rotating drive portion is the portion mounted around the fixed shaft 1 with the spacing 20, namely, the rotation sleeve 3, the mirror flange 4 fixed to the rotation sleeve 3 by press fitting, adhesion or the like, the magnet yoke 5, the inner magnet 6 and the outer magnet 7.
The rotating polygonal mirror 62 is installed on the rotor portion by inserting the rotation sleeve 3 into a center hole of the rotating polygonal mirror 62, covering them with the cap flange 16 and fixing the cap flange 16 on the rotation sleeve 3 by the screw 17. At the same timer an air reservoir 19 is formed to provide a damping effect in the axial direction of the shaft 1.
The stator portion of the rotation drive principally comprises the housing 2, the shaft 1 one end of which is fixed into the housing 2 by press fitting or the like, the toroidal stator core 8 fixed on the housing 2 by the stud 13 for fixing the stator core 18 and the screw 14, the substrate 10 supported by the substrate fixing stud 9 set on the stator core 8 and the magnetic field detecting element 11, for which a Hall element is suitable, mounted on the substrate 10.
On the housing 2, the housing adaptor 21 is fixed by the screw 23 and the substrate 10 is fixed by the collar 24 and screw 25.
The inner magnet 6 and outer magnet 7 are permanent magnets, and therefore a magnetic attraction force works between these magnets and the stator core 8 facing them. The magnetic attraction force prevents a relative movement of facing positions of the stator core 8 and these magnets in the axial direction of the fixed shaft 1.
That is to say, in FIG. 7, if the inner magnet 6 and outer magnet 7 shift upward, a component of the magnetic attraction force pulls down the rotor portion. If they shift downward, a component of the magnetic attraction force pulls up the rotor portion. Thus the inner magnet 6, outer magnet 7 and stator core 8 are held in a predetermined position facing one another by the influence of the magnetic attraction force. In other words, the inner magnet 6, outer magnet 7 and stator core 8 constitute a magnetic thrust bearing.
The magnetic field detecting element 11, for example, a Hall element, detects the flux of the outer magnet 7 to determine whether a north or south magnetic pole has passed in the rotation of the outer magnet 7.
A detection signal is transmitted to a control circuit (not shown in the figure) through a printed circuit on the substrate 10. Based on the detection signal, the control circuit determines the direction of the electrical current in the coils wound on the stator core 8. As a result, force is generated in such a direction that rotation continues by the mutual relationship between the inner magnet 6 and outer magnet 7. Like magnetic poles of the inner magnet 6 and the outer magnet 7 are arranged to face each other.
When the rotation sleeve 3 rotates, a high pressure air layer is generated around the fixed shaft 1, namely, in the space between the shaft 1 and the rotation sleeve 3. The rotation sleeve 3 is supported floating on the shaft 1, thus constituting the dynamic pressure air bearing.
The grooves 1-1 for generating the dynamic pressure are formed on the outer surface of the shaft 1 in the above example, but may be formed on the inner surface of the rotation sleeve 3.
The high pressure air layer maintains a fixed center of rotation of the rotor. For example, if the rotation sleeve 3 shifts to the right, the right side space in the rotation sleeve 3 is enlarged and the air pressure in the spacing is reduced. On the other hand, the left side space in the rotation sleeve 3 is narrowed and the air pressure in the space increases. The difference between air pressures in the right side space and left side space moves the rotation sleeve 3 to the left and finally it returns to its original position.
The rotating polygonal mirror 62 has the shape of a regular polygonal prism, with mirrors 63 on its side faces.
As illustrated in FIG. 6, an incident light beam from the laser or the like is reflected by a mirror surface of the rotating polygonal mirror 62. As the rotating polygonal mirror 62 rotates, the reflected light beam is gradually changed in direction, namely, deflected.
When the next mirror surface appears by rotation of the rotating polygonal mirror 62, the light beam is incident on it and deflected by this mirror surface in the same manner as by the previous mirror. Consequently the scanning with the reflective light beam is carried out within a certain angle range. The scanning speed depends on the rotation speed of the rotating polygonal mirror.
Another example of the conventional light deflector of this type is disclosed by Japanese Patent Application Unexamined Publication No. Sho. 62-231922 (1987).
However, in the light deflector according to the above-described conventional technique, high-speed rotation causes the temperature to rise severely which causes deterioration of rotation accuracy or an oscillation of the light deflector or even deterioration of performance of the whole optical apparatus.
In other words, this type of light deflector is required to rotate not only with high accuracy but also at high speed. To carry out the high speed rotation, it is necessary to make the motor, namely, the rotation drive portion work effectively so that the rise in temperature may be restrained to the minimum.
However, the conventional construction of the light deflector has limits to the capability to limit the heat generated in the rotation drive portion, and therefore dispersal of the heat should be considered.
The heat is principally generated in the stator core portion producing rotation torque, and accordingly the temperature rises considerably in this area. Sometimes a method for forced cooling of the light deflector from outside has been employed, but thereby the whole apparatus is inevitably bulky and expensive because of the cooling apparatus installed.
According to the construction of the light deflector shown in FIG. 7, generated heat stays around the stator core 8 due to the configuration of the magnet yoke 5 surrounding it, which prevents the heat from escaping, that is, impairs the heat radiation property. The temperature increases more and more as the rotation speed becomes higher and the performance and reliability of not only the light deflector but also peripheral apparatuses such as a laser oscillator or optical lens is damaged or impaired.